We sincerely thank the Society for Maternal-Fetal Medicine Publication Committee for its recent article that addressed noninvasive prenatal testing (NIPT). The document provides invaluable information and guidance for practitioners. A concept that was not addressed was test accuracy, which is itself a discrete measure of screening test validity. Accuracy is particularly important with regard to NIPT because commercial laboratories that offer these tests often advertise them as “>99% accurate” but provide no further explanation of how this figure is derived or what it means. Test accuracy simply is the fraction of all test results that correctly categorize patients regarding the condition of interest. This measure is expressed as the total of true-positive and true-negative test results divided by the total number of tests performed. For several reasons, accuracy is of limited value in assessing test performance.

In our experience, patients and clinicians commonly and mistakenly interpret the advertised accuracy as representative of the test’s positive predictive value. As the committee and we note, the positive predictive value of NIPT results rarely approaches the >99% value advertised for accuracy. The positive predictive value, which is calculated as the number of true-positive test results divided by the sum of true-positive and false-positive test results, is quite different from accuracy, although both vary with condition prevalence. We therefore suggest that pretest patient counseling directly address the issue of advertised accuracy vs actual positive predictive value. Such proactive discussions hopefully will dispel any misconceptions regarding accuracy and NIPT performance.